High Cr steel like stainless steel has been conventionally produced using ferrochromium as raw materials. In view of saving energy and securing low production cost, a so-called smelting reduction process is recently considered potentially attractive, which obtains high Cr molten metal directly using Cr raw ores (explanation will refer to "Cr ore" as example hereinafter). In the smelting reduction method, Cr ores, carbonaceous materials and others are supplied into a reduction furnace for directly obtaining high Cr molten metal.
For the smelting reduction method, there have been several proposals hitherto. One of them blows respectively O.sub.2 from bottom tuyeres and N.sub.2 from side blowing tuyeres at the same time as blowing O.sub.2 from a top lance. Another of them blows respectively O.sub.2 from the bottom tuyeres and O.sub.2 and N.sub.2 from the side tuyeres at the same time as blowing O.sub.2 from the top lance. The latter is disclosed in, for example, Japanese Patent Application Laid-Open 61-279,608 (1986).
However, each of the above mentioned methods has big problems that the Cr reduction speed is low and treatments take much time. With respect to the backgrounds of the prior art, following drawbacks will be taken up.
1. The reduction of Cr ore was progressed by action of C in carbonaceous materials after Cr ore had been molten in the slag, and the melting of Cr ore was assumed to determine Cr reduction. Therefore, important technical interests for shortening the treating time were focused to specifying of the slag composition. But Cr ore was inherently less to be molten, and it was limited to speed up the reduction by accelerating the melting of Cr ore. PA0 2. For accelerating the melting speed of Cr ore in the slag and the reduction speed, it was considered to make a post combustion of CO gas in the furnace and utilize the heat thereby. An existing method has been adopted, which blew O.sub.2 for the post combustion from the furnace upper part. If the post combustion ratio was increased, the temperature of an exhaust gas went up, but there has not been a technique which efficiently tansmitted a sensitive heat of the exhaust gas to the molten metal, and as a result, a heat transfer efficiency was lowered and the exhaust gas at high temperature was removed inevitably. The heated exhaust gas considerably damaged the wall refractories and those of an exhaust gas food. Therefore, in general it has been considered that the post combustion ratio could not be heightened so much. PA0 (1) A decarburization in the container of a converter type caused considerably loss of Cr by its oxidation (called as "Cr oxidation loss" hereinafter), and therefore although the smelting reduction was carried out in the converter, the decarburization had to depend on a vacuum system which was less in Cr oxidation loss such as RH-OB system. PA0 (2) The decarburization required much agitating gas, but on the other hand, the existing smelting reduction method did not want the agitating gas so much. Therefore, if the container of the same converter type was used, it had been considered that a furnace for the smelting reduction had to have a structure different from that of the decarburizing container. PA0 (3) For carrying out the treatments from the smelting reduction to the decarburization in the same container, the slag generated by the smelting reduction had to be removed, but an ordinary electric furnace could not remove the slag. PA0 (4) It took much time for the conventional smelting reduction method and decarburization method. Accordingly, if the both treatments were performed in the same container the whole treating time was made very long and lowered the productivity, and the furnace refractory would be considerably injured, so that the operation was made difficult. PA0 (1) As having been said above, it was assumed that Cr ore was reduced by the carbonaceous materials staying in the slag after Cr ore had been molten in the slag, but it has been found that almost all of the actual reductions were made by actions of C as the reducing material. Therefore, the reduction speed was determined by contacting of the molten metal to Cr ore heated at the high temperature, not by melting of Cr ore into the slag, so that the reduction speed could be effectively heightened by positively contacting of the molten metal to the ores, PA0 (2) It was basic concept in the prior art that the post combustion could not be largely heightened in view of the technical limit with respect to increasing of the heat transfer efficiency and consumption of the refractories. If O.sub.2 was blown so that the post combustion was mainly caused in the slag to forcibly agitate the slag, the heat transfer efficiency could be heightened effectively. Thus, by the high post combustion and the high heat transfer efficiency, the temperatures of the slag and Cr ore in the slag were heightened, and the reduction speed of Cr ore by C (in the molten metal) expressed by an under formula, may be raised effectively PA0 (3) The foregoing technique sometimes carried out the bottom blowing of O.sub.2 in a certain period or in a full term, but such blowing was harmful to the post combustion. That is, if O.sub.2 was blown from the bottom, CO gas was much generated in the molten metal and agitated the molten metal was agitated compulsively, and splashes of the molten metal reached the region of the post combustion, and since C reacted with O.sub.2, the post combustion was hindered. Therefore the bottom blowing had to be avoided, irrespective of a part and the full term of the reduction period. PA0 (4) From the above items (1) and (2), a certain strong agitation was necessary for performing the smelting reduction efficiently, and therefore it was possible to use the container (furnace) of the same structure as the decarburizing container. PA0 (5) If the decarburization was undertaken by a combination of the top blowing and the bottom blowing agitation under predetermined conditions, it was possible to provide the decarburizing treatment effectively in a short period of time with checking Cr oxidation loss. PA0 (a) In the smelting reduction, the combination of the bottom tuyere blowing and the side tuyere blowing of the agitating gases, the molten metal is positively diffused into a region of the slag where ore exists so as to accelerate the reduction of Cr ore by C in the molten metal. PA0 (b) In the smelting reduction, O.sub.2 for the post combustion is blown independently of decarburizing O.sub.2 for providing the post combustion ratio more than a predetermined level. The post combustion O.sub.2 is blown into the slag from the top blowing lance for forming the post combustion region therein, and the slag is agitated complusively by the side tuyere blowing gas, and a heat generated by the post combustion is caused to transfer to Cr ores. PA0 (c) In the smelting reduction, the side tuyere blowing gas and the bottom blowing gas are CO or inert gas, and O.sub.2 gas is not used because of avoiding hindrance of the reduction by C in the molten metal and the post combustion by the top-blowing O.sub.2. PA0 (d) In the decarburization, O.sub.2 is not blown from the bottom, since Cr oxidation loss is increased, and O.sub.2 is blown exclusively from the top lance only. O.sub.2 is not merely blown but O.sub.2 diluted with the inert gas is blown from the top lance for decreasing CO partial pressure around a fire point and accelerating the decarburizing reaction. At the same time, the inert gas is blown from the bottom tuyeres to forcibly agitate the molten metal for accelerating decarburization and controlling Cr oxidation loss. PA0 (a) blowing CO and/or inert gas from the bottom blowing tuyeres; PA0 (b) blowing CO and/or inert gas from the side blowing tuyere such that at least a part of the flowing gas contacts an upheaval of the molten metal made by gas blown from the bottom tuyere; PA0 (c) blowing the decarburizing O.sub.2 into the molten metal from the top blowing lance and O.sub.2 for the post combustion into the slag concurrently.
For finally obtaining the stainless molten steel from the Cr raw ore efficiently and economically, it is of course desirable to continuously carry out a smelting reduction and a subsequent decarburizion blowing in the same furnace. However, the prior art has not made studies on a method that the decarburizing treatment was carried out after smelting reduction in the same furnace for reasons as follows.
For dealing with the above stated problems of the prior art, the inventors made studies on the mechanisms of the smelting reduction and the decarburization, and the actual measures therefor, and subsequently they found following facts.
Cr.sub.2 O.sub.3 +C=Cr+CO